I used to run into this problem, however, I learned years ago, from my mom, that there is a solution. Now, when picking fruit, I also grab some of the under ripe fruit, which I was told, is often a great source of Pectin. (Lemon or orange zest, too, for me!)
Take a look at this post, as I go thru the process for how I make Beach Plum Jelly:
https://peakd.com/proofofbrain/@bluefinstudios/beach-baby-beach-baby
I always pick some underripe fruit, to include for more pectin.